Not entirely true, if you build a double arm tracker you can get exposure of over an hour long. The key is in the design. I am in the process of building one now (should be finished tomorrow once I get home from work) and based on my dimensions mine should track with exceptional results up to the first 30 mins then average results up to 60 mins and then there after exceptional results. The key is getting it to track at 15.04 degrees per min.
But you are right polar alignment is crucial.
If you want to build one, this is a great start http://www.davetrott.com/DoubleArmBarnDoor.html